My thought on this is that the format of *.wav is not correct. (Or the UC520 does not recognize it at least.)
For right now, I would suggest that you just record a new prompt using the prompt administrator on the UC520. From a "prompt adminstrator" phone, just call x397 (default extension for prompt adminstration), and record a new prompt. Then assign the new prompt to option 3.
If using CCA 3.0, the prompt admin extension, and list of adminstrators will be listed here: Configure>Telephony>Call Handling>AutoAttendant - Prompt Management tab.
Thanks,
Darren